i'm reliably informed they go down very
well on youtube so i thought yes we
should make a cat video
cheshire cat is a character from alice
in wonderland um and his cat is famous
for the fact that when it grins the cat
disappears but the grin stays behind
well i've often seen a cat without a
grin thought alice but a grin without a
cat it's the most curious thing i ever
saw in all my life so this is a paper
entitled observation of a quantum
cheshire cat in a matter wave
interferometer experiment although it's
a great tagline it actually fits as well
they have done this amazing thing so
just as your cheshire cat somehow the
smile gets separated from the cat
they've basically taken some property of
an elementary particle in this case a
neutron and separated it from the
particle so it's green is actually it's
it's a magnetic moment it's magnetism so
they have somehow managed to take a
neutron which has a magnetic moment and
they've separated the magnetism from the
particle neutrons are neutral in the
sense that they have no electric charge
associated with them but they do have a
magnetic moment in other words they do
behave like tiny little magnets probably
the way to think about it is if you take
a neutron and put it in a magnetic field
then the magnetic field and the
magnetic moment of the neutron will
interact with each other
and you know they'll attract and twist
each other around and so on because the
you know two magnets are repelling or
repulsing each other what have what has
this team done they have physically
separated the particle from its magnetic
moment
are you going to tell me more you're
just going to keep that okay i can keep
telling you do you want me to show you
how they did it i'd love you too okay so
it's a bit involved but i have some
pictures okay so we're in the world of
quantum mechanics which means basically
things behave in very strange ways um
but it means we have to sort of think in
some of the quantum mechanical terms and
in particular one of the things that's
going to feature in this experiment is
this concept of weak measurement which
we've also talked about in the past it's
basically trouble with quantum
mechanical systems is when you start
measuring them the properties of the
system change but there's this concept a
weak measurement which involves
measuring things sufficiently gently
that actually you don't change the
fundamental quantum mechanical
properties of the system so this is a
schematic view of what goes on so
basically you've got a beam of neutrons
coming in from the side over here and
these three blue blocks are what's known
as an interferometer and then we're just
going to detect what comes out the other
end the other thing you need to know is
that neutrons have a spin associated
with them
um they act like little rotating bodies
and in this weird quantum mechanical
world you can align them with their
spins in different directions typically
up and down or forwards and backwards
but you can align the spins you can play
around with the spins of these particles
professor when we talk about the spin of
a neutron should i be imagining like a
planet spinning on its axis or is it a
different property yes and no is the
short answer to that in that it behaves
very much like
a body spinning on its axis it has many
other same properties
but actually you are in this weirdly
fundamentally quantum mechanical world
where actually the particle you can't
think of it as a little sphere solid
sphere
if it's an electron it's completely
point-like for example but you can still
have a spin associated with it so you
shouldn't be thinking about these things
as solid spheres you've got to think
about them as fundamentally different
sorts of entity but nonetheless they
still have a property which is analogous
to the rotation of a little sphere so
we've got a beam of neutrons heading
into this apparatus and they've been
oriented so that their spins are all
pointing upwards
okay which is just to say you know that
you can think of them going around
clockwise or anti-clockwise and you can
think of that as the spin pointing up or
the spin pointing downwards and it's
just convenient to show that rotation as
an arrow this thing is an interferometer
which basically splits that beam of
neutrons into two
now this is where we get into the weird
quantum mechanical world because
actually you can't think of one neutron
going this way and another neutron going
this way in this weird quantum
mechanical world one particle actually
follows both parts we're getting into
this properties of quantum mechanical
things that they behave a bit like waves
in that we split our beam so our
particles go both ways and then when
they recombine coming out the other side
we can actually generate these
interference effects what we can
actually think of the neutron as a wave
going this way and a wave going this way
and depending on exactly how we orient
the apparatus sometimes those waves will
add up in a constructive way and so
we'll get a big signal other places
they'll
add up in a destructive way and we'll
get very little signal in other words by
messing around with the path difference
we can create one of these interference
patterns that sometimes we get lots of
neutrons coming and then we change the
apparatus a little bit and suddenly we
get very few neutrons coming so it's a
classic interference experiment it's one
of the classic ways of showing
the this fundamental property of quantum
mechanics that things that we think of
as particles neutrons actually have
wave-like properties as well
so that's the the beginning of the setup
okay the next thing we need to do is
change the apparatus a bit so the next
thing we do
is we introduce one of these things
called a spin rotator it takes the spin
and twists it round so instead of
pointing upwards suddenly this spin
points backwards
and we put a spin rotator in one beam
that rotates the things one way we put a
spin rotator in the other beam which
rotates the particles the other way
so now we've got our beam of neutrons
going these two different paths but
actually one of them has it's been
aligned in one direction and the other
in completely the opposite direction but
again remember these are actually still
the same neutrons more or less that one
neutron could be going along both parts
and can have it spin simultaneously
twisted one way and twisted the other
way
yes it's all getting very very weird i'm
afraid
it'll get worse before it gets better
okay now when we recombine these two
beams you get no interference effect at
all
and that's because one of the properties
of these particles is they're what's
known as an orthogonal state which means
that basically they don't interact with
each other when they recombine and so we
don't get these interference effects
just because of the way we've set them
up with completely
spins pointing in completely opposite
directions so what do we get coming out
instead just just get a beam of neutrons
coming out and we can mess around with
the parts all we want and we're never
going to see some places where things
add up or some places where they don't
add up because they don't interfere
anymore just because of what we've done
with the spins
all right
whoa
yeah it's getting worse all right i've
got three of these there's another one
to come after this all right here's page
two so the same apparatus as before
we've just introduced one more element
here
which is this little box which just
basically says is the spin aligned to
that pointing in that direction is the
spin aligned to the left
right a spin identifier yes and if it
spins a line to the left it lets it
through if it spins a line to the right
it doesn't let it through it doesn't
make any difference we still get the
same kind of thing coming out the other
side we'll probably get a few rather
fewer neutrons because some of them be
rejected by it but basically we don't
see any interference effects so now we
can ask okay so which path
the neutrons that actually get through
here which path did they follow and
actually in a non-quantum mechanical
world the answer that is kind of obvious
right because all the ones over here
were pointing to the left all the ones
over here were pointing to the right
this one only lets things that point to
the left through and therefore clearly
anything the only thing that's going to
come out the other side is something
that followed this path here that's
obvious in the in the rational normal
world but in the world of quantum
mechanics where these two things are
kind of tangled up together
it's nowhere near as obvious
but you can do an experiment to figure
out which path the neutrons actually
followed and this is where we have to
get into this weak measurement thing we
can't do anything very kind of robust
because we'll mess up the quantum
mechanical system but what you can do
is you can put a little attenuator in on
one in one beam or the other so
something which just blocks out a few of
the neutrons but it's only a few percent
it's not like you know half of them or
anything like that like a crocodile just
pulling in the odd wheel of boost
exactly and you know most of the most of
the herd gets by but but it just stops
one or two and what you find is when you
put it in this upper beam here indeed
the signal level drops that you get out
when you put the same attenuator in the
lower beam nothing happens and so that
immediately tells you that the the
neutrons that we're seeing that have
made it through clearly follow this
upper path here well i could have told
you that but exactly
as i say that's kind of the common sense
part all right now we get to the
non-common sense part i told you what
we're going to try and show is that the
particle and its magnetic moment have
been separated from each other okay
and one way you can do that is by let's
get to the last picture here
we can put a magnet in
okay and then what will happen if you
think about what happens here in the
outside put a magnet here in the upper
beam which is where we know the neutrons
that we're actually detecting are going
we've got neutrons going through the
lower beam nothing happens to them at
all when we put a magnet in that
actually starts pulling this spin around
right because we've got the now the the
magnetic moment of the neutron interacts
with the magnetic field and that twists
the spin around which means that
the the particle instead of being
set up in this state where it's purely
kind of pointing that way it'll be
mostly pointing that way but a little
bit pointing that way
and again it's one of these weird
quantum superposition things but at
least there's some probability
associated with these neutrons that they
might actually have their spin aligned
in the same direction as the lower beam
but not all this magnet
again it has to be a weak measurement so
it's a very weak magnetic field which
just means a few percent of them will
may have you know this a few percent
probability that it's actually in that
state rather than that so what should
happen now is that some fraction of the
neutrons are now aligned in the same
direction their spins are aligned in the
same direction on top and bottom which
was the condition we needed for
interference to start again okay so we
should start seeing fringes again start
seeing the interference effects just
little weak ones though a little weak
into yeah because we haven't done much
but we should start seeing just a little
bit of interference okay
when you put the magnet in the upper
beam
nothing happens
you get no interference coming out the
other side so for whatever reason these
neutrons have not in any way been
affected nothing has happened to them
but it gets weirder because if we put
the magnet in the lower beam instead of
the upper beam
then we start seeing interference again
so it looks like these particles in the
upper beam their magnetic moments are
going the lower root
and we can mess with their magnetic
moments in the lower root and then when
we combine the whole thing together then
suddenly we start seeing interference
again their magnetic moments are going
along the bottom the particles go along
on the top we showed we've shown right
just from the previous experiment with
the attenuator we've shown that the
particles we're actually detecting here
have all gone through the upper root but
what we've also shown from this
experiment is that those same particles
magnetic moments are following the lower
root
i think they've stuffed something else
this smacks of a mistake
you think they just go how they they had
the experiment their own way up or
something i don't know maybe the top
magnet wasn't working oh it's the same
magnet so you just move it from the top
to the bottom
so it's weird right that when you put
attenuators in that tells you that the
particles that you're detecting over
here are following the upper root when
you put magnets in that shows you that
actually their magnetic moments are
traveling along the lower route
and so that's this whole thing of the
cheshire cat right you've separated the
cat from its smile in this case you've
separated the neutron from its magnetic
moment it turns out there might actually
be a use for it too do you want a use
for it let me guess cryptography no not
cryptography for change the use for it
is you might have you might want to
study some very fundamental property of
a particle
but the fact that it has a magnetic
moment masks the very subtle effects
you're looking for because you know what
the experiment you're trying to do ends
up interacting with the magnetic moment
of the particle and rather than the very
subtle property of the particle you're
actually after
potentially if you can use this kind of
apparatus to separate physically
separate the particle from its magnetic
moment you can now do your experiments
on the particle the naked particle
without the magnetic moment and from
that actually learn very fundamental
properties of nature that you're not
going to be able to find when the two
are sat on top of each other